1. Field of the Invention
The invention concerns a starter/generator for an internal combustion engine, especially an engine of a motor vehicle.
2. Description of the Related Art
The invention concerns a starter/generator for an internal combustion engine, especially an engine of a motor vehicle.
Motor vehicles and other vehicles outfitted with internal combustion engine generally require an electric starter to start the engine, as well as a generator to supply electrical consumers and to charge a battery, which furnishes the electrical energy needed for the starting, among other things. Since the dawn of automotive engineering, starter and generator have generally been two separate electric machines, which are especially adapted to their particular functions. A starter must produce relatively high torques at relatively low engine rotary speed and is therefore traditionally geared up relative to the engine. It is not constantly running, but is only coupled to the engine for the starting process. On the other hand, a generator runs constantly with the engine and achieves high rotary speed at relatively high transmission ratio.
The combination of these two functions in one and the same electric machine is a worthy goal, since in this way one of the two traditionally necessary machines can be economized.
One possibility of combining the starter and generator despite the different requirements of the two machines is known from G. Henneberger: "Electrical Engine Equipment," Vieweg, Braunschweig 1990, p. 98-103. According to this proposal, the electric machine, which is an invertor-controlled rotary-field machine--does not start the internal combustion engine itself, but rather accelerates a flywheel (in the beginning decoupled from the engine). Upon reaching a sufficiently high speed, the flywheel is coupled to the crankshaft of the engine by means of a frictional coupling. The rotational energy stored in the flywheel then turns over the engine. In generator mode, the electric machine is permanently coupled to the engine by the frictional coupling. This solution has the advantage that the powers and torques of the electric machine are similar during starting and in generator mode. The disadvantage, however, is the large mechanical load of the frictional coupling when engaging the fast-running flywheel, resulting in wear on the coupling, among other things, as well as a dead time prior to each starting process, which is needed to accelerate the flywheel.